Automobile chassis parts are subjected to repeated impact load, torsional load, etc. during working of course and also during vehicle operation, so high strength and high fatigue characteristics are required. Impact load, torsional load, etc. sometimes become large loads which reach the plastic region of the material, so, in particular, the fatigue characteristics in the high stress amplitude, low cycle region (frequency of fractures of 105 or less) are emphasized.
For example, regarding the automobile chassis part of an automobile axle beam, PLT 1 proposes a method of imparting fluid pressure to the inside surface of a tubular worked member (for example, steel pipe) while press-forming it and an irregular cross-section cylindrically shaped axle beam which is obtained by that method.
In this axle beam, to secure sufficient fatigue characteristics, the steel pipe is quenched, annealed, or otherwise heat treated for hardening after press-forming it so as to improve the fatigue characteristics and strength of the part to a desired level.
If performing such hardening heat treatment, the part becomes higher in cost. Furthermore, sometimes heat treatment causes the part to change in shape and makes additional correction necessary and sometimes the part softens and makes additional strengthening means (for example, treatment for surface hardening etc.) necessary.
For this reason, an automobile chassis part which has sufficient fatigue characteristics which can be produced without heat treatment after press-forming has been desired in industry.
Further, as an automobile chassis part which is excellent in fatigue characteristics, PLT 2 proposes a part which is made from steel to which Nb and Mo are added compositely. An Nb and Mo composite steel becomes harder in the surface layer by work hardening after bending. Further, there is little drop in hardness at the time of annealing for removal of internal stress performed for improvement of the fatigue characteristics, so the fatigue characteristics are excellent.
True, it is believed that if annealing after press-formation, sufficient fatigue characteristics are obtained.
However, in the state as press-formed, ferrite is present, so a large number of micro voids which become starting points for fatigue cracks after press-forming are liable to be formed. Further, with a low cycle region of fatigue, the stress amplitude becomes much greater than the yield stress of the ferrite phases, so the ferrite phases easily slide and fatigue damage locally occurs. Therefore, it cannot be believed that sufficient low cycle fatigue characteristics are obtained.